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7. PROGRAMMABLE LOGIC CONTROLLERS (PLCs)

•CONTROL - Using artificial means to manipulate the world with a particular goal.

•System types,

•Continuous - The values to be controlled change smoothly. e.g. the speed of a car as the gas pedal is pushed

•Logical - The values to be controlled are easily described as on-off. e.g. The car motor is on-off (like basic pneumatics).

Note: All systems are continuous but they can be treated as logical for simplicity.

•Logical control types,

•Conditional - A control decision is made by looking at current conditions only.

e.g. A car engine may turn on only when the key is in the ignition and the transmission is in park.

•Sequential - The controller must keep track of things that change and/or know the time and/or how long since something happened.

e.g. A car with a diesel engine must wait 30 seconds after the glow plug has been active before the engine may start.

Note: We can often turn a sequential problem into a conditional by adding more sensors.

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•Advantages of PLCs are:

-cost effective for complex systems

-flexible (easy to add new timers/counters, etc)

-computational abilities

-trouble shooting aids

-reliable

-easy to add new components

•Ladder logic was originally introduced to mimic relay logic.

7.1 BASIC LADDER LOGIC

•The PLC can be programmed like other computers using specialized “languages.”

-Ladder Logic - a programming technique using a ladder-like structure. It was originally adopted because of its similarity to relay logic diagrams to ease its acceptance in manufacturing facilities. The ladder approach is somewhat limited by the lack of loops, etc. (although this is changing).

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POWER NEEDS TO FLOW THROUGH

THE INPUTS TO THE OUTPUTS

- Mnemonic - instructions and opcodes, similar to assembly language. It is more involved to program, but also more flexible than ladder logic. This will be used with the hand held programmers.